Field of the Invention
The present invention relates to a method of making a mimeographic printing plate through which ink can be passed when an image is printed and, more particularly, to a mimeographic plate making method suitable for reproducing the half-tone in printing the image.
Description of the Background Art
There is a method of mimeographic printing for which a mimeographic printing plate is constituted of a support having a number of minute apertures where ink can be passed and a membrane covering the support to be impermeable to the ink. The ink is passed through the minute apertures and the printing areas where the membrane is taken off. Non-image areas on the mimeographic plate are defined by the shape of the remaining membrane on the support. An example of this printing method with such a mimeographic plate is a mimeograph.
There has been proposed a mimeographic printing method with a mimeographic plate, in which the support is a screen of woven fabric or a non-woven fabric having minute apertures, the membrane is a thermoplastic resin formed thereon, and the plate material is heat-sensitive. An original to be reproduced carries an image drawn, or letters written, with a black ink including carbon black. Also, an original to be reproduced may carry a photocopy of an image copied by an electrostatic press copying machine. The original is superimposed on a sheet material prepared for a mimeographic plate. A flash is emanated from a flash lamp and applied to the rear of the plate material. Black portions of the illuminated original image absorb the light to generate heat, which in turn melt and shrink the resinous membrane facing the black portions, so as to form printing areas defined around the unmelted portions of resinous membrane, in correspondence with the black portions of the original image. There is an article commercially available including the same kind of plate material and equipment for making mimeographic plates, called PRINT GOKKO (trade name) manufactured by Riso Kagaku Industries Co., Ltd.
To use this method, the original image must be black and have a sufficiently high optical density. No mimeographic plate can be directly made from an original having a color image. The conventional mimeographic plate making method is also disadvantageous because a flash lamp is used, which is expensive. To make a good mimeographic plate, a flash from the flash lamp is emanated a number of times. Melting the resinous membrane of the plate material requires the generation of a large amount of heat. Illumination of the flash lamp once, however, is insufficient for making a reliable mimeographic printing plate.
There is another mimeographic printing plate making method in which heating of the heat-sensitive plate material is done by a thermal head. An example of this method is conducted by use of an article PRINT GOKKO CD MASTER B6/B5 included in a series of merchandise PRINT GOKKO, and by this method a mimeographic plate can be made by use of a thermal printer of a word processor. This method dispenses with any flash lamp. An image scanner is adapted to be used with the thermal printer and reads even an original color image, which is converted into an electric signal, and the thermal printer is supplied with the electric signal to treat the plate material, which is made into mimeographic plates of three colors, cyan, magenta and yellow. The three mimeographic plates are sequentially applied to full-color or multicolor printing with three-color ink so as to reproduce a color image on paper.
Mimeographic plates as made with a thermal printer reproduce either an image without gradation or a binary value image having only letters and lines. The mimeographic plates, however, cannot reproduce the half-tone of the original image. There is a method of reproducing a pseudo half-tone image taking advantage of the integrating function of human eyes regarding a space, by forming a plurality of dot matrices, and by changing an areal proportion of dots as recorded in the matrices. This is a method of reproducing an areal gradation of a binary value. Examples of this method are a dither pattern method and a dot pattern method.
The binary value areal gradation reproducing method, however, is disadvantageous because it has a considerably low degree of resolving power. Whereas a method of applying 4.times.4 matrices is inferior in a mere 17 step gradation, it has a lower quality from a resolving power 1/4 as high as the former method. Reproduction of a photographic image should be conducted by use of an areal gradation of at least 32 steps, and preferably an areal gradation of 64 steps or more. However, when the number of steps in the gradation are larger, the resolving power would be lower, or inferior in fineness for a reproduced image.